Presently, the manufacture of coiled PVC tubes is carried out by means of the extrusion of a ribbon of soft material and of a rib of a relatively stiff material. These two components are co-extruded in a coil on a series of flexible rotary shafts mounted on a main fixed shaft for keeping them on track.
On their turn, the flexible rotary shafts are mounted on a central shaft by means of a plurality of supporting elements with an inclination defining the pitch of the rib on the tube. During the formation of the coiled tube, every pitch causes a partial overlapping of the components (“ribbon” and “rib”), thus provoking, together with high temperatures, the welding of the overlapping areas and therefore the forming of the coiled tube. The presently available systems, however, have some drawbacks.
In fact, the pitch of the rib on the coiled tube can be changed only by manually varying the position of the supporting elements by rotating them around a substantially longitudinally symmetric axis of the main central shaft and by fixing them in a new angular position by means of allen keys to the main central shaft. Furthermore, once a value is determined for the pitch of the rib, for varying this pitch, the apparatus must be stopped and the supporting elements must be rotated and then locked in a new angular position to define the new desired pitch. All this clearly involves a remarkable loss of time and a consequent loss of production.
Moreover, there is an increasing need of ribs having different pitches and stiffness but placed on the same tube. In fact, the areas of the tube provided with less stiff ribs and with a lower pitch can be used with relatively soft joints to be directly coupled with stiff tubes. This may involve a more cost effective process due to the complete absence of joints to be associated to the coiled tubes to allow their assortment on stiff joints.